1. Field of the Invention
This invention relates to a method of reducing the redox ratio (FeO/Fe2O3) of molten glass, and the glass made thereby, and more particularly, to a method of introducing oxygen into molten glass having a low iron content to oxidize the iron in the ferrous state (Fe++) to reduce the redox ratio.
2. Discussion of the Presently Available Technology
Solar collectors and solar mirrors use solar energy to heat a fluid, e.g. as disclosed in U.S. Pat. Nos. 4,224,927 and 5,253,105, or to convert solar energy to electrical energy. In general, the solar collectors have a cover plate to pass the solar energy, to reduce heat loss due to convection, and to protect the photovoltaic cells of the electric power generating solar collectors, and the solar mirrors have a glass substrate to pass the solar energy to a reflective coating and reflect the solar energy back through the glass substrate to direct the solar energy to a designated area. Of particular interest in the following discussion are the glass cover plates and the glass substrates.
As is appreciated by those skilled in the art, the glass cover plates used for photovoltaic cover plates, and the glass substrates used for solar mirrors preferably above 380 nanometers (“nm”) of the electromagnetic spectrum have a high transmission, e.g. above 90% in the visible and the infrared (“IR”) range, and a low absorption, e.g. below 2% in the visible and the IR ranges. As is appreciated by those skilled in the art, the particular visible and IR range of the electromagnetic spectrum, and the peak transmission varies depending on the semi-conductor material of the photovoltaic cell. For example and not limiting to the discussion, for a silicon photovoltaic solar cell, the preferred visible and IR wavelength range is 380-1200 nm, and the peak transmission is at about 900 nm.
Generally, in the manufacture of flat glass, glass batch materials are melted; the molten glass is fined and homogenized, and the fined homogenized molten glass is formed into a flat glass ribbon by controllably decreasing the temperature of the molten glass as it floats on a molten metal bath. During the fining of the molten glass, gas bubbles are removed from the molten glass by additions of ingredients to the batch materials, and/or by moving gases, e.g. carbon monoxide and oxygen through the molten glass, e.g. see U.S. Pat. Nos. 2,330,324 and 6,871,514. The batch materials for making glasses having high transmission, and low absorption, in the visible and the IR range of the electromagnetic spectrum have no additions of colorants. As is appreciated by those skilled in the art, additions of colorants to the batch materials have been used to, among other things, reduce the transmission and increase the absorption in the visible and IR range of the subsequently formed glass. Glasses having high visible and IR transmission are usually referred to as low iron glasses. U.S. Pat. Nos. 5,030,593; 5,030,594, and 6,962,887 disclose the making of low iron glasses that are almost colorless by processing raw glass batch materials that have a very low content of total iron expressed as Fe2O3, e.g. less than 0.020% by weight (hereinafter also referred to as “wt %” or “wt. %”). Iron contents of less than 0.020% by weight (200 parts per million (hereinafter also referred to as “ppm”)) in batch materials are referred to as tramp iron because the iron is not added to the batch material but is present as an impurity in the ingredients of the batch material.
Even though the iron content is low in low iron glasses, it is also preferred to reduce the weight percent of ferrous iron (Fe++) in the glass to maximize the transmission, and minimize the absorption of the glass in the visible and IR range of the electromagnetic spectrum. As is appreciated by those skilled in the art, iron in the ferric state is a less powerful colorant than iron in the ferrous state and shifts the transmittance spectrum of the glass toward yellow and away from the usual green-blue effect of the ferrous iron in glass. Stated another way, increasing iron in the ferric state while decreasing iron in the ferrous state, increases the transmission, and decreases the absorption of the glass in the visible and the IR range. One technique to reduce the weight percent of ferrous iron in the glass is to include cerium oxide in the glass batch materials because cerium oxide in the glass “decolorizes” the glass. More particularly, cerium oxide is not a colorant in glass, but is a powerful oxidizing agent in glass, and its function in decolorized glass is to oxidize the iron in the ferrous state (Fe++) to iron in the ferric (Fe+++) state. Although cerium oxide is useful to decolorize the remaining traces of ferrous iron, the use of cerium oxide has limitations, e.g. but not limiting to the discussion, when the glass is to be used as cover plates for electric power generating solar collectors and as glass substrates for solar mirrors. More particularly, exposing low iron glass cover plate having cerium oxide to the sun has a solarizing effect on the glass, which results from the photo-oxidation of Ce+++ to Ce++++ and the photo-reduction of Fe+++ to Fe++. As is appreciated by those skilled in the art, the solarization effect of cerium and the photo-reduction of Fe+++ to Fe++ reduces the transmission, and increases the absorption, of the glass in the visible and the IR range of the electromagnetic spectrum, which reduces the power generation of the solar cells.
As can now be appreciated, it would be advantageous to provide a low iron glass that has low levels of iron in the ferrous state (Fe++) and does not have the limitation of the photo-reduction of iron in the ferric state (Fe+++) to iron in the ferrous state (Fe++).